Sierra 3.9:1 Front Differential

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It’s been a few weeks since my last blog and for good reason. My project has a new home. I had to knock down walls and do some pretty scary electrics, but the work is coming to an end. The next job is a heavy duty chassis table.

More good news…

I’ve finally unearthed a Sierra 3.92:1 Front Differential. They were fitted to the Sierra DOHC 2.0i GLS/X 4×4 01/15/1990 – 15/02/1993 (non-cosworth). I’ve bought 2 previous diffs that claimed to be 3.92 but this one really is!!

It’s the real thing! I was seriously beginning to believe that they were an internet myth. I’d even been to the local Ford dealer and searched dozens of microfiche over several hours.

As you can see by the drawings top left, I intend to replace the Cosworths’ sump assembly with a custom built unit. The assembly will also include a strengthening plate.

In this build, the diff will sit in front of the engine and not alongside.

The Sierra 3.92:1 tag reads:

V90BB AA
C9P5A CG02

I will confirm the precise ratio when I rebuild it.

The FINIS number for the 3.92:1 is 1661121

1 1634747 Axle Assy – Front Diff to 15/1/1990 | 3.62 from 15/1/1990 to 15/2/1
1 1661121 Axle Assy – Front Diff from 15/1/1990 to 15/2/1993 | DOHC 2.0 3.92
1 6478336 Axle Assy – Front Diff from 3/1/1990 to 15/2/1993 | Cosworth 2.0 3.62
2 1637137 Shaft – Intermediate to 15/2/1993 |
3 1632648 Tube – Intermediate Shaft to 15/2/1993 |
4 1627953 Gasket to 15/2/1993 |
5 1627954 Gasket to 15/2/1993 |
6 1634748 Bearing Assy – Ball to 15/2/1993 |
7 1637272 Gear – Speedometer to 15/2/1993 |
8 1634753 Circlip to 15/2/1993 |
9 1542801 Circlip to 15/2/1993 |  40 x 1.85MM
10 6802339 Circlip to 18/1/1993 | Cosworth 2.0  25 x 1.6MM
10A 6008347 Circlip to 15/2/1993 | Cologne V6to 15/2/1993 | D  38 x 1.8MM
10A 6586453 Circlip to 15/2/1993 | Cosworth 2.0
11 1643701 Cover – Intermediate Shaft to 15/2/1993 |
12 6152992 Bearing – Speedo Driven Gear to 5/1/1993 |
13 1630829 Pin – Roll to 15/2/1993 |  4 x 20MM
14 6028927 “O” Ring to 5/1/1993 |
15 1629528 Gear – Speedometer Driven to 15/2/1993 |
16 1634970 Shaft – Front Axle to 15/1/1990 |from 3/1/1990 to 15/2/1993 |
16A 6478335 Shaft – Front Axle from 3/1/1990 to 15/2/1993 | Cosworth 2.0  RH
16A 6672254 Shaft – Front Axle from 3/1/1990 to 15/2/1993 | Cosworth 2.0  LH
17 1627944 Joint And Stub Shaft Assy to 15/1/1990 |from 15/1/1990 to 15/2/1993 |
17A 6672253 Joint And Stub Shaft Assy from 3/1/1990 to 15/2/1993 | Cosworth 2.0
18 1632763 Shaft Assy – Drive to 15/2/1987 | Cologne V6 2.8
18 1659240 Shaft Assy – Drive to 15/2/1993 | Cologne V6 2.9 EFIfrom 3/1/1
19 1627950 Kit – Drive Shaft Boot Repair to 15/9/1991 |from 15/9/1991 to 15/2/1993 |
19 6672258 Kit – Drive Shaft Boot Repair from 15/9/1991 to 15/2/1993 | Cosworth 2.0
19A 1638320 Kit – Drive Shaft Boot Repair to 15/9/1991 |from 15/9/1991 to 15/2/1993 |
19A 6672257 Kit – Drive Shaft Boot Repair from 15/9/1991 to 15/2/1993 | Cosworth 2.0
20 1436268 Washer – Flat to 15/2/1993 |  10.5 x 20 x 2.5MM
21 6036438 Washer – Flat to 15/2/1993 |  20.2 x 40.3 x 5MM
22 6104837 Bolt – Hex.Head to 15/2/1993 |  M8 x 20MM
23 1613366 Bolt – Hex.Head to 15/2/1993 |  M10 x 55MM
24 6152647 Screw – Socket Head Cap to 15/6/1988 |  M8 x 16MM
24 1650759 Screw – Socket Head Cap from 15/6/1988 |  M8 x 16MM
25 6036437 Nut – Special Hex. to 15/2/1993 |  M20 x 1.5MM
26 1660443 Bracket to 15/2/1993 | Cosworth 2.0
27 1660444 Bracket to 15/2/1993 | Cosworth 2.0

Ford Sierra Front Diff Diagram

Here it is… All bright and shiny……..

Ford Sierra Front Differential 3.92:1

Ford Sierra Front Diff

I have two rear differentials. One is a 3.92:1 tripod diff (push in shafts) and the other is a 3.62:1 Lobro LSD unit (bolt on shafts) from an Escort Cosworth. I have read many articles on rebuilding these diffs, but I think I will leave the job of swapping the crown and pinion gears between these two, to the professionals. If a front and rear rebuild kits turns up cheap on Fleebay then I’ll hand them over at the same time. I’ve searched long and hard for these now rare and increasingly expensive diffs and I’m not about to ruin them!

For more info on the Sierra front diff click here

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